The present invention relates to a photosemiconductor device mounted structure and, more particularly, to a photosemiconductor device mounted structure in which a photosemiconductor device used for an optical communication optical module is mounted on a substrate.
As a method of mounting a photosemiconductor device on a substrate, for example, Japanese Patent Publication No. 5-53315 discloses a method of forming an Sn film at a predetermined position on a substrate by vacuum deposition and fusing the Sn film and an Au electrode of the photosemiconductor device. This method requires cumbersome positioning adjustment of the photosemiconductor device and cannot meet a recent demand for cost reduction of an optical communication module.
Among modules using photosemiconductor devices, one in which a photosemiconductor device is bonded to an Si substrate with a solder of AuSn or the like is proposed. According to the characteristic feature of this bonding method, a photosemiconductor device can be located at a bonding position defined by a pad on the Si substrate without adjustment because of the self-alignment effect of the surface tension of a fused solder piece, and attracts attention as an effective means for cost reduction of a module. This optical module structure is shown in, e.g., "A Surface Mount Type Optical Module for Subscriber Networks" (proceedings of the 1995 IEICE Conference, SC-1-12) by Kurata et al.
FIG. 5 shows a conventional photosemiconductor device mounted structure using solder bonding. As shown in FIG. 5, thin AuSn solder pieces 105 are placed on pad-like Au electrodes 104 formed on an Si substrate 103 to correspond to Au electrodes 102 of a photosemiconductor device 101. The Au electrodes 102 and 104 are aligned with each other and the photosemiconductor device 101 is placed on the Si substrate 103. After that, the AuSn solder pieces 105 are fused and solidified, so that the photosemiconductor device 101 is mounted at high precision.
However, the prior art shown in FIG. 5 has problems as follows.
The first problem is as follows. Since the coefficient of thermal expansion of the photosemiconductor device and that of the Si substrate are largely different from each other, when the solder (AuSn or the like) is fused and solidified, distortion and stress occur in the photosemiconductor device. In the worst case, cracking or fracture occurs in the photosemiconductor device. This shortens the service life of the photosemiconductor device to degrade the reliability of a module using the photosemiconductor device.
Furthermore, in the case of a photosemiconductor device array, its size becomes larger than that of a photosemiconductor device since photosemiconductor devices equal in number to the number of channels are formed in it. When the photosemiconductor device array is used, not only distortion or stress but also warp or deflection occurs in it as the solder is fused and solidified. This degrades not only the service life of the photosemiconductor device array but also optical coupling of the optical fiber and the active layers (in the case of light-emitting devices) or absorption layers (in the case of light-receiving devices) of the respective channels of the photosemiconductor device array, thus degrading the reliability of the module.
The second problem is that the photosemiconductor device cannot operate stably. This is because distortion or stress occurs in the active layer (in the case of a light-emitting device) or absorption layer (in the case of a light-receiving device) of the photosemiconductor device. In particular, when a distributed feedback LD (Distributed Feedback Laser Diode; DFB-LD) is used as the photosemiconductor device, with the conventional method, distortion or stress occurs in the diffraction grating of the active layer, and stable single-mode operation is difficult to perform.